1. Field of the Invention
This invention relates to rotating machinery and the seals employed therein which can deteriorate and/or cause damage to opposing surfaces during occasional rubbing contact due to misalignment, distortion or vibrations.
2. Description of the Prior Art
A persistent and costly problem for steam turbines and other compressible fluid rotating machinery utilizing seal teeth to minimize leakage is the harmful effect of rubs between stationary and rotating parts. Such rubbing increases the clearance of the seal teeth allowing greater leakage losses and harmful effects on efficiency. Beyond the effect of increased clearance area is the increase in leakage flow coefficient caused by mushroomed and rounded seal teeth. Another harmful effect is the weakened surfaces and grooves caused in the opposing surfaces that are contacted by the seal teeth such as shafts and bucket covers and shrouds. A final harmful effect is the effect of forces applied to the rotating shaft that cause vibration and interfere with balancing procedures.
A variety of steps have been taken to prevent or minimize the problems itemized above. These include improved alignment, corrections of distorted parts, retractable packings, increased seal clearance and refined balance. These are all helpful but cannot completely eliminate occasional seal rubbing and the resultant disadvantages described above.
It has also been proposed to provide a continuous protective coating on rotating parts to protect them against grooving by the seal teeth.
While this may protect the rotating parts, it has an adverse effect on the seal teeth.
For instance, in Martin et al patent #5,211,535, the protective coating 38 is a 360.degree. deposit on the shaft 14. As such, it cannot act as a cutting tool. Rather, when rubbing occurs, it must abrade the teeth to remove them. This is essentially a melting process which creates highly undesirable conditions on the teeth and the rotor. Martin et al states in column 3 lines 13, 14, 15, "the inner shaft 14 includes a layer 38 of abrasive resistant material which is applied to the outer surface of the shaft in the area swept by seal teeth 36." That, of course, is the full 360.degree. portion of the shaft opposing the teeth 36.